Paper machine belt

ABSTRACT

A paper machine belt that includes a cross-machine direction supporting layer providing strength in the cross-machine direction and at least two additional layers. The two additional layers include polymeric material and at least one of the additional layers that includes the polymeric material is positioned on one side of the cross-machine direction supporting layer and at least another of the additional layers that includes the polymeric material is positioned on an opposite side of the cross-machine direction supporting layer. The cross-machine direction supporting layer substantially includes cross-machine direction yarns loosely bound together with very fine machine-direction yarns. A ratio of mass of the cross-machine direction yarns to the machine direction yarns being at least 160:1, and mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation in Part of InternationalApplication No. PCT/GB03/00830, filed Feb. 24, 2003, and claims priorityof Great Britain Patent Application No. 0204310.7, filed on Feb. 23,2002. Moreover, the disclosure of International Patent Application No.PCT/GB03/00830, filed Feb. 24, 2003, and International PatentApplication No. PCT/GB03/00824 filed Feb. 24, 2003 are expresslyincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to paper machine process belts andparticularly, but not exclusively, to belts for transferring and/orsmoothing the paper web within, to and/or from a press section of apaper machine.

2. Discussion of Background Information

Transfer belts are used for carrying a paper web through a portion of apaper machine so as to eliminate open draws in which the paper web isunsupported and is thus likely to break. When the web breaks the papermachine must be shut down and consequently this constitutes a seriousproblem to the papermaker. Such transfer belts tend to have a smoothsurface which can aid smoothing of the paper sheet and provide anextremely uniform pressure distribution in a nip with no basecloth mark.The surface should nevertheless provide for eas sheet release.Furthermore, the belt in use should be water impermeable.

In EP 1127976, a transfer belt comprises a base support having a layerof thermoplastic material formed thereon. A batt of fibrous material islocated on top of this thermoplastic material. The whole structure isthen heated in order to allow the thermoplastic material to migrate tothe surface. It is common for transfer belts, such as that described inEP 1127976, to suffer from the drawback of edge curl, i.e., inwardcurling of the edges of the belt. In severe cases this can lead to thebelt folding over which would, at the very least, mark the roller andcause misalignment and damage to the paper sheet.

Edge curl is caused by differential contraction properties associatedwith different materials used within the belt structure, as well asvarious finishing processes. For example, polyurethane film has a muchgreater shrinkage than the yarns of a woven structure and so when a filmlayer attached to a woven base cloth is partially melted, on cooling, itexerts a contractive force on the base structure, resulting in curling.

The present invention has been made from a consideration of this.

SUMMARY OF THE INVENTION

According to the present invention there is provided a paper machinebelt having paper machine a layer for providing strength in thecross-machine direction (hereinafter “the cd supporting layer”) and twoor more additional layers, wherein at least two of the additional layersinclude a polymeric material and wherein at least one of the layers ofpolymeric material is provided on one side of the cd supporting layerand at least another of the layers of polymeric material is provided onthe obverse side of the cd supporting layer.

For example, surprisingly, the provision of a similar mass of polymericmaterial on either side, i.e. face of the cd supporting layer, whichgives the cross-machine strength and rigidity for the belt, results in abalancing effect insofar as the anticipated differential thermalshrinkage properties of the polymeric layers are concerned, so as toyield a belt with no edge curl. Different polymeric materials havingdifferent shrinkages may be used in combination, so that further toexposure to thermal energy, the shrinkage forces are balanced,eliminating edge curl.

Ideally at least one layer of polymeric material is provided on thepaper facing surface of the belt. The polymeric material is ideallyprovided as a film, but may, for example, alternatively be provided assinterable material or as a fibrous material. This layer, in thefinished product, is ideally water-impermeable.

Preferably some, and ideally all, of the layers of polymeric materialpreferably have a Shore hardness in the range from 30A to 75D, andideally have a hardness of substantially 90 Shore A. Ideally, the weightof each polymeric layer is approximately 400 g/m². The thickness of eachlayer of polymeric material is ideally in the range from 0.4 to 1.0 mm.The preferred polymeric material is a thermoplastic polymer, such as apolyether based polyurethane, ideally in film form.

Polymeric materials having different colors may be used. For example, ifa laminate of two polymeric materials were to be used in which theindividual lamina had different colors then as the outer layer wearsaway the lower layer (having a different color) would become exposed.This would act as wear indicator. Alternatively, the use of differentlycolored polymeric materials might be useful for providing a guiding barmark across the belt width. Pigments and additives may be included inthe polymeric material as desired, such as photochromic pigments and/orultra-violet fluorescent material.

The cd supporting layer providing the cross-machine strength is ideallya structure made up of a plurality of cross-machine direction yarns,preferably multi-strand (e.g. multifilament or cabled) yarns, which arelaid in close proximity to one another. However, in order to give easeof handling, the cross-machine direction yarns may preferably be looselybound together with very fine machine-direction yarns. For example, themachine-direction yarn diameter may be in the order of 0.1 mm andselected for pliabilty, compared to the stiff cross-machine directionyarn with a diameter of approximately 0.5 mm. The ratio of the mass ofthe cross-machine direction yarns to machine direction yarns is ideallysubstantially at least 160:1. The whole layer providing cross-machinestability has a weight of approximately 200 g/m². Ideally, this layerincludes approximately 9 to 15 cross-machine direction yarns/cm,preferably 12 cross-machine direction yarns/cm.

It was found that this quasi-nonwoven cd supporting layer embeddedbetween the layers of thermoplastic polymer, further to heat treatmentand calendering, results in a laminated product with much improvedmacro-level pressure uniformity, due to the fact that there was far lesschance of sheet marking, as is typical of substrates containing wovensubstrates with pronounced warp knuckles.

In addition, the belt preferably includes a machine direction (md)supporting layer to be needled on the roll side of the structure, toprovide strength and md stability. This md supporting layer may be inthe form of a woven, knitted or molded perforated membrane, for exampleof the type described in EP 0285376. However, this supporting fabricideally includes an array of strong, stable, spirally wound, machinedirection yarns. Layers of fibrous batt can also be needled in order tohold the yarns in position and to provide a coherent structure. Theprovision of additional batt on the roll side can also offer better wearresistance. In a preferred embodiment, the machine roll side layer ismade up of spirally wound, machine direction, 0.2. mm/2 ply/3 cabled,polyamide yarns, with approximately 7 to 12 yarns/cm. There isapproximately 50-800 g/m² of polyamide batt fiber in the range of 3 to67 dtex needled thereon. This whole layer preferably has a weight in-therange from 450 to 480 g/m².

The spirally wound layer of md yarns with batt needled thereon is thepreferred supporting substrate for a number of reasons. Firstly, markingdue to cd/md yarn cross-over knuckles, as exist in woven substrates, issubstantially eliminated.

Secondly, shrinkage upon heat-setting of a fabric generally takes placemainly in the cross machine direction because the machine directionlength is held constant between the two rollers on the stretcher. Thespirally wound layer is more yielding than a woven structure in the cd,due to there being only batt between the md yarns, which does notprevent the yarns from bunching together in order to conform to theshrinkage requirements of the thermoplastic film located thereon.

It is possible to use, in place of the cd and md supporting layers, asingle base structure, which acts as both a cd and md supporting layer.This may be, for example, a woven material or possibly a nonwoven or afilm. A preferred example is a double layer woven fabric with polymericmaterial on both faces.

Examples include laminates and integrally woven multiple layer bases.Yarn sizes would typically be from 0.2 mm to 0.6 mm in diameter.

A layer of batt fiber, preferably in the range from 3.1 to 44 dtex,would ideally be needled to this woven base structure.

The structure as a whole may additionally include separate layers ofbatt fibers. The batt is needled to any other layers so as tomechanically inter-lock them together, as well as providing a higherlevel of pressure distribution. The batt used preferably has a weight inthe range from 50 to 800 g/m² and ideally in the order of 300 g/m². Thetotal belt thickness is normally betveen 2.4 and 3.2 mm, with an averageweight of between 2600 and 3300 g/m².

The preferred structure of the invention includes at least five mainlayers, which working from bottom to top include:

-   -   1) an md supporting structure,    -   2) a thermoplastic film or films,    -   3) a cd supporting structure,    -   4) a fibrous batt, and    -   5) further thermoplastic film or films, ideally wherein the mass        of thermoplastic material of layer (2) is substantially the same        as that in layer (5).

The belt is preferably made endless, but could feasibly include a seam.

The whole structure is consolidated through needling at various stagesduring the manufacturing process. As a result of the needling stage, thesurface is not entirely smooth as there are around 1 to 200 batt fibersper square cm, and preferably 10-100 batt fibers per square cmprotruding through to, and in some instances standing proud of, thepolymer surface. These provide for good sheet release. The fibers mighttake the form of loops, that is the middle of the fiber has been pushedthrough with both ends still remaining locked within the structure. Ifdesired, the protruding fibers may be removed.

Further to the needling process, the entire structure is then exposed tosufficient thermal energy to cause the lower melt point, thermoplastic,polymeric film to melt. This melted polymer bonds the structuretogether, embedding the cd supporting layer and part of the batt in amatrix of molten polymer and forms a very smooth and well definedimpermeable surface., which is resistant to de-lamination. The belt isthen smoothed with a cold polished cylinder. There is no need to grindthe finished product, which is advantageous because this is extremelydifficult to achieve in the case of low melt polymeric elastomers.

In an alternative preferred structure the order of layers “1” and “2”hereinbefore described is swapped around, such that the structureincludes at least five main layers, which working from bottom to topinclude:

-   -   1) a thermoplastic film or films,    -   2) a supporting structure providing machine-direction stability,    -   3) a structure providing cross-machine stability,    -   4) a fibrous batt, and    -   5) further thermoplastic film or films, wherein the mass of        thermoplastic material of layer (1) is preferably substantially        the same as that in layer (5) to minimize edge-curl. Such an        arrangement helps prevent batt loss and assists with ease of        cleaning.

Another aspect of the invention includes a paper machine belt thatincludes a cross-machine direction supporting layer providing strengthin the cross-machine direction and at least two additional layers. Thetwo additional layers include polymeric material and at least one of theadditional layers that includes the polymeric material is positioned onone side of the cross-machine direction supporting layer and at leastanother of the additional layers that includes the polymeric material ispositioned on an opposite side of the cross-machine direction supportinglayer. The cross-machine direction supporting layer substantiallyincludes cross-machine direction yarns looselv bound together with veryfine machine-direction yarns. A ratio of mass of the cross-machinedirection yarns to the machine direction yarns being at least 160:1, andmass of the polymeric material of the additional layer on the one sideof the cross-machine direction supporting layer is substantially thesame as a mass of the polymeric material of the additional layer that isprovided on the opposite side of the cross-machine direction supportinglayer.

Further aspects of the present invention include at least two additionallayers that can include the polymeric material with different polymers.At least one of the additional layers can include the polymeric materialprovided on a surface of the belt and structured and arranged to supporta paper web. Moreover, the additional layer that includes the polymericmaterial, provided on the surface of the belt, can be water-impermeable.Additionally, at least one of the additional layers that includes thepolymeric material can have a Shore hardness of from 30A to 75D. Athickness of the additional layer that includes the polymeric materialcan be from 0.4 to 1.0 mm. At least one of the additional layers ofpolymeric material can comprise a thermoplastic material.

Further aspects of the present invention include the thermoplasticmaterial can be a polyether-based polyurethane. The cross-machinedirection supporting layer can exhibit different shrinkage to theadditional layers of polymeric material. Additionally, at least two ofthe additional layers can include the polymeric material with differentcolors. Moreover, the cross-machine direction supporting layer caninclude a plurality of multi-strand cross-machine direction yarns.Additionally, the belt further can include a machine directionsupporting layer. The machine direction supporting layer can include anarray of spirally wound machine direction yarns. Moreover, the beltfurther can include at least one layer of batt.

Yet another aspect of the invention includes a paper machine belt thatincludes a cross-machine direction supporting layer providing strengthin the cross-machine direction and at least two additional layers. Thetwo additional layers include polymeric material and at least one of theadditional layers that includes the polymeric material is positioned onone side of the cross-machine direction supporting layer and at leastanother of the additional layers that includes the polymeric material ispositioned on an opposite side of the cross-machine direction supportinglayer. The cross-machine direction supporting layer substantiallyincludes cross-machine direction yarns bound together withmachine-direction yarns. A ratio of mass of the cross-machine directionyarns to the machine direction yarns being at least 160:1, and mass ofthe polymeric material of the additional layer on the one side of thecross-machine direction supporting layer is substantially the same as amass of the polymeric material of the additional layer that is providedon the opposite side of the cross-machine direction supporting layer.

Another aspect of the invention includes a method of making a papermachine belt. The method including providing a cross-machine directionsupporting layer having strength in the cross-machine direction,positioning one of at least two additional layers that include apolymeric material on one side of the cross-machine direction-supportinglayer, positioning at least another of the additional layers thatinclude the polymeric material on an opposite side of the cross-machinedirection supporting layer, and heating the at least two additionallayers and the cross-machine direction supporting layer. Additionally, amass of the polymeric material of the additional layer on the one sideof the cross-machine direction supporting layer is substantially thesame as a mass of the polymeric material of the additional layer that isprovided on the opposite side of the cross-machine direction supportinglayer.

Further aspects of the method can further include calendering the atleast two additional layers and the cross-machine direction supportinglayer. Moreover. at least two of the additional layers can include thepolymeric material include different polymers. Additionally, theadditional layer that includes the polymeric material, provided on thesurface of the belt, can be water-impermeable. Also, at least one of theadditional layers can include the polymeric material which has a Shorehardness in the range from 30A to 75D. Additionally a paper machine beltcan be manufactured according to the above-noted method.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood, aspecific embodiment thereof will now be described by way of example onlywith reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic cross-section of a transfer smoothing belt inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to FIG. 1, a transfer and smoothing belt 17 for use in thepress section of a paper machine includes an endless loop having fivelayers 11-15.

The supporting layer 11, includes spirally wound machine direction yarns16 into which batt has been needled to hold the yarns 16 in position. Inthis embodiment the machine direction yarns include three pairs of yarnstwisted together.

The second layer 12, located on layer 11, itself includes two individuallayers of thermoplastic polyurethane each having a weight of 400 g/m²and being 0.4 mm thick. On heating these two polyurethane layers, asingle homogeneous layer is formed which bonds and partially impregnatesthe md supporting fabric 11 and the adjacent upper cd supporting layer13.

Layer 13 includes a quasi non-woven structure made up of multifilament,cross-machine direction yarns and extremely fine machine directionyarns, for loosely holding the cross-machine direction yarns inposition. This layer has a weight of approximately 195 g/m². The mass ofmaterial ratio of cross-machine direction yarns to machine directionyarns is approximately 160:1. This layer provides cross-machinedirection strength and rigidity.

A layer 14 of batt is located above the cross-machine directionsupporting structure 13 to facilitate inter-locking of the variouslayers by needling. The batt material preferably has a weight in theorder of 300 g/m².

The final layer 15 of thermoplastic material is ideally identical to theinner thermoplastic material layer 12. On heating, the constituentpolyurethane layers flow and bond the structure together, embedding thetop cd supporting layer and part of the batt 14, in a polymeric matrix,to form a single homogenous layer. The belt is cured at a surfacetemperature of approximately 200° C. with a dwell time of 5 minutes. Itis then calendered at 1 to 40 KN/m at a temperature of less than 200° C.

Surprisingly, by embedding a cd supporting structure between layers ofmelted thermoplastic polymeric material, a balance of contractive forcesis achieved. That is, the relatively stiff, high bending moduluscross-machine direction orientated yarns, placed in a positionrelatively near to the top plane of the fabric, between the meltedpolymeric material, can successfully balance the contractive forces ofthe melted polymeric material, so as to yield a belt with no edge curl.

From recent in-house trials on a pilot machine it has been found thatthe belt described above gives excellent transfer at 2000 m/min and thesurface has been found, using Martindale Abrasion testing methods, toexhibit good abrasion resistance. In particular, the belt was found notto suffer from edge curl.

It is to be understood that the above-described embodiment is by way ofillustration only. Many modifications and variations are possible.

1. A paper machine belt comprising: a cross-machine direction supportinglayer providing strength in the cross-machine direction; and at leasttwo additional layers that include polymeric material and at least oneof said additional layers that includes the polymeric material ispositioned on one side of the cross-machine direction supporting layerand at least another of said additional layers that includes thepolymeric material is positioned on an opposite side of thecross-machine direction supporting layer, wherein the cross-machinedirection supporting layer substantially comprises cross-machinedirection yarns loosely bound together with very fine machine-directionyarns, a ratio of mass of the cross-machine direction yarns to themachine direction yarns being at least 160:1, and mass of the polymericmaterial of the additional layer on the one side of the cross-machinedirection supporting layer is substantially the same as a mass of thepolymeric material of the additional layer that is provided on theopposite side of the cross-machine direction supporting layer.
 2. Thepaper machine belt according to claim 1, wherein at least two of saidadditional layers that include the polymeric material comprise differentpolymers.
 3. The paper machine belt according to claim 1 wherein atleast one of the additional layers that include the polymeric materialis provided on a surface of the belt and is structured and arranged tosupport a paper web.
 4. The paper machine belt according to claim 3wherein the additional layer that includes the polymeric material,provided on the surface of the belt, is water-impermeable.
 5. The papermachine belt according to claim 1 wherein at least one of the additionallayers that includes the polymeric material has a Shore hardness of from30A to 75D.
 6. The paper machine belt according to claim 1 wherein athickness of the additional layer that includes the polymeric materialis from 0.4 to 1.0 mm.
 7. The paper machine belt according to claim 1wherein at least one of the additional layers of polymeric materialcomprises a thermoplastic material.
 8. The paper machine belt accordingto claim 7, wherein the thermoplastic material is a polyether-basedpolyurethane.
 9. The paper machine belt according to claim 1 wherein thecross-machine direction supporting layer exhibits different shrinkage tosaid additional layers of polymeric material.
 10. The paper machine beltaccording to claim 1 wherein at least two of the additional layers thatinclude the polymeric material have different colors.
 11. The papermachine belt as claimed according to claim 1 wherein the cross-machinedirection supporting layer comprises a plurality of multi-strandcross-machine direction yarns.
 12. The paper machine belt according toclaim 1 wherein the belt further comprises a machine directionsupporting layer.
 13. The paper machine belt according to claim 12,wherein the machine direction supporting layer comprises an array ofspirally wound machine direction yarns.
 14. The paper machine beltaccording to claim 1 wherein the belt further comprises at least onelayer of batt.
 15. A method of making a paper machine belt comprising:providing a cross-machine direction supporting layer having strength inthe cross-machine direction; positioning one of at least two additionallayers that include a polymeric material on one side of thecross-machine direction supporting layer; positioning at least anotherof said additional layers that include the polymeric material on anopposite side of the cross-machine direction supporting layer; andheating the at least two additional layers and the cross-machinedirection supporting layer, wherein a mass of the polymeric material ofthe additional layer on the one side of the cross-machine directionsupporting layer is substantially the same as a mass of the polymericmaterial of the additional layer that is provided on the opposite sideof the cross-machine direction supporting layer.
 16. The methodaccording to claim 15 further comprising: calendering the at least twoadditional layers and the cross-machine direction supporting layer. 17.The method according to claim 15, wherein at least two of saidadditional layers that include the polymeric material comprise differentpolymers.
 18. The method according to claim 17, wherein the additionallayer that includes the polymeric material, provided on the surface ofthe belt, is water-impermeable.
 19. The method according to claim 15wherein at least one of the additional layers that includes thepolymeric material has a Shore hardness in the range from 30A to 75D.20. A paper machine belt that is manufactured according to the method ofclaim
 15. 21. A paper machine belt comprising: a cross-machine directionsupporting layer; and at least two additional layers that include apolymeric material and at least one of said additional layers thatincludes the polymeric material is positioned on one side of thecross-machine direction supporting layer and at least another of saidadditional layers that includes the polymeric material is positioned onan opposite side of the cross-machine direction supporting layer,wherein the cross-machine direction supporting layer substantiallycomprises cross-machine direction yarns bound together withmachine-direction yarns, the ratio of a mass of the cross-machinedirection yarns to the machine direction yarns being at least 160:1, anda mass of the polymeric material of the additional layer on the one sideof the cross-machine direction supporting layer is substantially thesame as a mass of the polymeric material of the additional laver that isprovided on the opposite side of the cross-machine direction supportinglayer.